Sunday, October 16, 2011

Changes in birth rates by age cohort, 1980 to 2008

It's no secret that fertility rates in the Western world outside of Israel and the United States are below replacement level, and the populations in some developed countries like Japan and Germany have already began contracting. The US, at 2.06 children per woman, would be treading water if not for the country's net immigration level.

Sifting through new Census data, I came upon a file containing data on fertility rates by several characteristics of the mother. Although whites, blacks, and Asians have all become less fecund over the last thirty years [or not--please see Hail's comment below], their declines have been almost exactly offset by a rapid increase in the size of the more procreative Hispanic population, such that the total fertility rate in the US today is right where it was in 1980.

Also of interest are shifts in fertility patterns by age that have occurred over the last three decades. The following table shows the percentage changes in birth rates by the mother's age from 1980 to 2008:

Age

Change

10-14

(45.5%)

15-19

(21.7%)

20-24

(10.5%)

25-29

+1.9%

30-34

+60.4%

35-39

+136.9%

40-44

+151.3%

45-54

+250.0%

Women are having children later than they did a generation ago. The bulk of all birthing is still done by women in their 20s (see below), so the large percentage changes among women in their teens and thirties is less impactful on the whole that it might appear at first glance.

Now for the percentage of total births by age range of the mother in 1980 and 2008. Data are not available for women 40+ in the earlier period, so I estimated using simple algebra to get the total for the 40-44 and 45-54 age ranges, and then split that number proportionally in accordance to the most recent year in which births for women in those age ranges are recorded:

Age

1980

2010

Under 20

15.6%

10.4%

20-24

33.9%

24.8%

25-29

30.7%

28.2%

30-34

15.2%

22.5%

35-39

3.9%

11.5%

40-44

0.7%

2.5%

45-54

0.03%

0.2%

A plurality of births are now to women aged 25-29, a change from three decades ago, when women aged 20-24 gave birth most frequently. In 1980, women in their 20s accounted for 64.6%--nearly two-thirds--of all live births, while women in their 30s accounted for 19.1%, or less than one-fifth of the total. Today, at 53.0% of all births, women in their 20s account for just over half. Women in their 30s now account for 34.0%--over one-third. If these trends continue, in another generation it will be more common for babies to be born to mothers in their 30s than to mothers in their 20s.

"whites, blacks, and Asians have all become less fecund over the last thirty years"

Incorrect, according to the CDC. Interestingly, White non-Hispanics have a higher TFR today than in the 1980s. (Or the same level, if only counting births wherein both parents are white). Blacks: same level. Asians: slightly-higher TFR.

"The US, at 2.06 children per woman, would be treading water if not for the country's net immigration level"

I don't think we would be treading water. We would be declining without immigration because the fertility of women whose grandparents were born in the US is well below 2.06 on average. Even blacks whose grandparents were born in the US are below 2.00. One in eight blacks is either an immigrant, or has a parent or grandparent who was not born in the US, like Barack Obama.

As Hail notes, it could be we are transitioning because the personality types that are willing to limit fertility were able to do so and now there are fewer of those personalities in the childbearing years. As time passes this could mean permanent higher fertility because birth control selection has left us with a pool of people who are inherently disposed to want more kids, not merely culturally accustomed to it.

Some conservative churches have growing factions of pastors and laity that are against birth control including the formerly merely conservative Mo. Synod who are getting more catholic by the day, some even calling themselves evangelical catholics.

It looks like that's an amateur mistake on my part with re: to birth rate vs. fertility rate. According to the US Census, birth rates for all three groups have declined over that period (as the link in the article shows), but that's of course in part a function of a larger percentage of each of those groups being outside of their reproductive years (because people are living longer).

Do you happen to have the statistic on the probability of infertility or birth defect based on the female's age? Also does the ethnicity play a role? How effective are fertility treatments?

I read somewhere that after the age of 30, the birth defect rate goes up by 6-10% until females hit the age 40, when it steadily declines 1-2% until menopause. At age 40 most women are only 10-15% fertile.

I'm amazed that the government doesn't teach these statistic as part of sex education and career planning.

The WebMD article linked to in the post is the best I'm able to do. I was actually talking to a girl who's a senior in high school and taking a pregnancy planning course (yes, such electives actually exist) and she said that the students are told that under 17 and over 35 are high risk ages for pregnancy, so at least that's something.